Placing holes (i.e., gaps) in the audio portion of a radio transmission is necessary in order to allow interruptions of transmissions in quasi-duplex communication systems. In addition, information may be transmitted in the holes. However, some applications for these holes may require that their duration be as long as 40 mS, or more. Holes of such duration may significantly degrade the quality of the audio signal. When these holes occur at a relatively high frequency (e.g., 3 times per second) the audio degradation may become unacceptable to most listeners. Therefore, some hole filling (i.e., replacement of the information removed to create a hole) may become necessary to preserve the quality of the audio.
Most hole-fill algorithms fix drop-outs or noise bursts. Those systems generally have very short time frames (e.g., 8 mS) and thus are relatively easy to fix. However, where longer hole durations are used, better hole-fill methods are required.